Gravity has had a profound effect on the development of life on Earth over millions of years and has shaped the anatomy and physiology of human beings. Exposure to microgravity has been shown to affect the body causing it to undergo a reduction in heart size and blood volume, impaired balance control, changes in nervous system sensitivity, decreases in bone and muscle mass, and reduction of the immune function. Astronauts in space during short or long-term missions have demonstrated these physiological changes, known as space deconditioning, which may lead to undesirable health consequences and to operational difficulties, especially during emergency situations. Physiological deconditioning is a critical problem in space, especially during long-term missions. Despite physiological deconditioning, a future involving microgravity environments is quickly becoming a reality.
With the recent advent of space tourism and with longer space missions planned, greater numbers of astronauts will work and live in low-gravity environments, and the need to understand the in-flight and post-flight consequences of this lack of gravity will become more significant. The physiological adaptations have proven to be less problematic while still in space, but become more pronounced after an astronaut returns to Earth. Many different types of countermeasures have been developed over the years, ranging from specific diets to heavy exercise protocols that must be performed daily by the astronauts during a space mission. Ideally, the best way to counteract the consequences of space deconditioning would be the use of artificial gravity through centrifugation or other biomechanical stressors for periods of time during microgravity exposure.
Among the countermeasures currently under testing, daily exercise in space seems to be the most complete, since it can have an important positive impact on bone demineralization, muscle loss and cardiovascular deconditioning. The mechanical unloading affects the musculoskeletal system even in short-duration space flights. It has been reported that after only two weeks in space, muscle mass can decrease by 20%. For missions of three to six months duration this loss of muscle mass can rise to 30%, especially affecting postural muscles. The decrease in bone mass is also of great concern to space physiologists and physicians, as the normal processes of bone formation and resorption are disturbed, favoring a loss of bone tissue. This process begins almost immediately upon introduction into microgravity, and can range between one and two percent of bone mass loss per month. One of the first responses to space flight is the shift of blood and body fluids towards the upper body, with subsequent adaptations occurring over a few days to lower overall blood volume through activation of several mechanisms. It is upon return to Earth that the cardiovascular deconditioning raises concerns by producing significant orthostatic intolerance and decreasing aerobic performance.
Astronauts participating in space shuttle missions, which are usually two weeks long, exercise for approximately 30 minutes per day. Astronauts who live on the International Space Station (ISS) for much longer periods of time are required to exercise for approximately two hours per day. Each astronaut's exercise routine is monitored, and can be adjusted if necessary based on his or her monthly fitness assessment. If astronauts are scheduled to perform a spacewalk, their exercise routines may be altered or restricted.
Understanding how to combat the negative effects imposed by microgravity could allow researchers to apply an exercise routine to terrestrial rehabilitation protocols that would decrease the required rehabilitation time. The negative effects discussed above occur at an accelerated rate in space in comparison to on Earth, allowing researchers to collect data faster.
One in six Americans has osteoporosis or early signs of the disease. Even though the causes behind osteoporosis and space induced bone loss are different, the treatments may be similar.
The human body experiences similar physiological changes as astronauts after encountering shattered bones. When a bone is broken or fractured the healing process is very slow or incomplete because the blood supply is often damaged. This may lead to amputation and/or a longer recovery time. When the cast is removed, the weakness resulting from muscle atrophy is very apparent. The rate of major amputations has changed throughout the course of combat operations in the Afghanistan Theater of Operations (ATO). This rate of amputation suggests an increasing demand on the healthcare continuum, from the battlefield to long-term rehabilitation centers. Again, the reasons these negative effects occur are different but the solution can very much be the same. Daily exercise using range-of-motion and muscle-strengthening exercises is necessary for people to combat stiffness and regain strength. Applying a differential pressure not only adds stress to the body's systems; it forces the blood to flow in its most healthy and natural way.
The effectiveness of exercise protocols and equipment for astronauts in space are unresolved and still under discussion. Prior studies indicate that all exercise in space to date has lacked sufficient mechanical and physiological loads to maintain preflight musculoskeletal mass, strength, and aerobic capacity. Researchers have been pairing exercise with a Lower Body Negative Pressure (LBNP) Box. The LBNP Box is a sealed device into which the user is partially inserted. A seal near the waist allows a vacuum to be applied to the device, thus creating a lower relative pressure on the user's lower body. This lower pressure helps pull bodily fluids toward the feet. Exercise within an artificial environment (LBNP Box) has been shown to counteract microgravity-induced deconditioning during terrestrial testing. A recent study on the addition of a treadmill to an LBNP Box has demonstrated that it is able to simulate the physiological and biomechanical features of upright exercise. However, the treadmill's mechanical design lacks mobility and is both large and heavy, making it unsuitable for space flight.
An exemplary LBNP Box is described in T. Russomano et al., “Development of a lower body negative pressure box with an environmental control system for physiological studies”, Advances in Space Research 38, 6; 1233-1239 (2005).